1. Field of the Invention
The present invention relates to an improvement of a zero correction method for a pressure detecting device of an injection molding machine.
2. Description of the Related Art
It is generally known that a pressure sensor for detecting a pressure, which acts on a screw of an injection molding machine, is provided between a thrust bearing for rotatably supporting the screw and a housing of an injection mechanism. In order to transmit the pressure acting on the screw to the pressure sensor accurately, in this case, the sensor is held under a pre-load between the thrust bearing and the housing of the injection mechanism. Accordingly, the pressure sensor undergoes a fixed compressive strain which is attributable to the pre-load. Thus, in a pressure detecting device of the conventional injection molding machine, a load equivalent to the pre-load is applied to the pressure sensor even when no substantial load is acting on the screw.
To cope with this, the output of the pressure sensor is read immediately before the start of a metering or injection stage, the read value is subtracted, as an error caused by an initial compressive strain in the pressure sensor, from a detected value obtained afterward in a sampling stage, and feedback control of an injection pressure is effected with the resulting remainder regarded as the pressure which acts on the screw. One such zero correction method for a pressure sensor is described in a Japanese Patent Application published as KOKAI 61-266219.
According to this method, zero correction is carried out on the assumption that no load acts on the screw at the point of time immediately before the start of the metering or injection stage. At the time just before the start of the metering or injection stage, however, resin exists in an injection cylinder, and sometimes influences the reception of pressure by the screw. Accordingly, the screw cannot always be in a stable state, so that accurate zero correction cannot be guaranteed.
Disclosed in a Japanese Patent Application published as KOKAI 2-63713, moreover, is a technique in which an injection force transmitting member, which connects a drive source and a screw, is provided with a pressure sensor without a preload thereon. This pressure sensor also requires zero correction, since it is subject to changes in properties attributable to temperature variation and aging. During actual operation for the correction, however, some external force of an unknown magnitude, such as a resin pressure, influences the reception of pressure by the screw, thus making it difficult to ensure accurate zero correction.